More than 20 years of Farm Journal Test Plot research has revealed a thing or two about corn and soybean row spacings—and the learning continues.

In 2012, Farm Journal Associate Field Agronomist Missy Bauer wrapped up a three-year study looking at corn row spacings in the eastern Corn Belt. In central Illinois, Farm Journal Field Agronomist Ken Ferrie continued his work to learn more about how hybrids respond to plant populations, plant densities and stress of all sources.

Located across southern Michigan, Missy Bauer replicated row spacing test plots at three locations every year in fields varying from no-till to conventional tillage to irrigated.

"We launched this effort to see if my more northern location had any difference compared with the work done by Ken in Illinois," Bauer says.

Using a Great Plains Yield-Pro planter, Bauer’s test plot crew planted corn in twin rows (on 30" centers) and 30" rows at five populations: 29,000; 33,000; 36,000; 39,000 and 42,000 plants per acre. In-season stand counts and ear counts were recorded and each plot was harvested with a calibrated yield monitor and grain cart outfitted with scales. Then the data was analyzed by management zones.

In Bauer’s test plots it didn’t take long to realize that twin rows demand management changes.
"For example, with in-furrow starter fertilizer, the rate needs to be calculated per foot of row. So if you were applying 3 gal. in-furrow on 30" corn, you need to apply 6 gal. in-furrow for twin rows to achieve the same expected response," Bauer says.

In 2012, increasing the rate of in-furrow fertilizer from 3 gal. to 6 gal. in twin rows increased the yield response by 8.4 bu. When paired with 2x2 placement, the 6 gal. in-furrow rate outyielded the 3 gal. rate by 4.6 bu.

When it came to nitrogen management, higher populations also mean an increase in rates.

"We learned that when you have higher populations, you need to follow up with the necessary nitrogen," Bauer says. "If you are changing to twin rows from 30" rows you need to continue your nitrogen application practices, such as sidedress. Stay on top of your application window because twin rows will canopy and close faster."

In a tighter row canopy, especially with increased plant populations, scouting and disease management should be top of mind. Because the rows close sooner, more water is captured in the canopy and it’s slower to dry out, which sets the scene for disease.

At harvest, Bauer also recommends scouting for stalk quality. A fungicide can help improve standability.

"As we increase populations, we will have weaker stalk quality," she says. "Farmers should monitor fields to keep the plant standing for harvest."

Averaging the multiple locations by year, at the same population the twin rows outyielded the 30" rows in a range from 1.6 bu. to 9.5 bu.

"Overall, our work builds on what Ken has learned in that proper management of narrow rows can bump up yields," Bauer says. "Looking at the test plot results, you have to dial in the proper population for the row spacing to maximize yields. The default thinking about narrow rows is push populations higher.

Instead, farmers should be thinking what’s best for that hybrid in that field. Dialing in population is key."

Averaging the multiple locations by year, at the best population for the twin rows, led to the narrow rows outyielding the 30" rows by 13 bu. in 2010; 6 bu. in 2011 and 7.9 bu. in 2012.

"There’s a misconception that twins need higher population, which can be true some times but not always," Bauer says. "Seeing how population drives yield response reinforces the opportunity to variable-rate populations in twin rows. Everything we have learned about variable rate will come into play to get population dialed in for each management zone."

Like all Farm Journal Test Plots, Bauer harvested her plots using a calibrated yield monitor and grain cart with scale package, in this case from DigiStar.

Capture the sunlight. The advantage to narrow rows is tied to the plant spacing in the field.

"By using light sensors in the field, we’ve seen that maximum plant growth and maximum yield occurs when 97% of the sunlight is intercepted by the plant instead of the soil surface," Ferrie says. "Once that level of light has been intercepted, there is no advantage to pushing plant densities because sunlight is the limiting factor."

When measuring the inches between the plants in the row, as population climbs, the plants are closer together, but row spacing can maximize the available space. For example, in Bauer’s plots at 42,000 plants per acre the in-row plant spacing was measured at 10.6" in twin rows and 5.3" in 30" rows. The corn plants are actually spaced closer together at 29,000 population in 30" rows than at 42,000 in twin rows.

"We know that spacing effects how the corn plants perceive stress, so the more space for each plant the less potential stress," Ferrie says. "So our goal becomes to not overcrowd plants and still achieve maximum light interception as quickly as possible."

Another key variable is the hybrid. Building on the lessons learned in row-spacing studies, AgriGold Hybrids partnered with Ferrie to conduct test plots looking at hybrids categorized by ear type—fixed and flex.

"Every hybrid flexes, but when characterizing them they can be grouped into flex ear hybrid, which have tremendous ability to flex ear size to fill in lower plant populations, or the more fixed, determinant hybrids, which tend to need higher plant populations to maximize yield," Ferrie explains. "In our plot studies, when under stress, the fixed hybrids were more likely to maintain ear size, while flex hybrids can flex backward."

Reflecting on years of data, Ferrie says there is a sweet spot in narrow rows where maximum light capture occurs.

"Row spacing and plant populations affect how fast we reach the maximum light capture. I’ve found that the lowest population where we can still maximize light capture will net the biggest yield response. That’s the advantage to narrow rows," Ferrie says. "Once you go past that sweet spot the yield penalty can be stiffer than I would have imagined.

Narrow rows and higher populations require farmers to stay on top of nitrogen management and the window to sidedress.

To build on knowing how ear type effects yield, Ferrie expanded the study to include leaf structure of the hybrid. "We planted four hybrids: one flex ear and one fixed ear both with an upright leaf structure, and one flex ear and one fixed ear both with a pendulum leaf structure," Ferrie says.

Because of the drought conditions in 2012, only preliminary data was able to be collected.

"We ran out of water before other stress factors could be evaluated.

However, there were big differences in how the twins handled drought and how the leaf structures handled drought. When water isn’t such a limiting factor, the upright leaf did better. But ear genetics trumped the effect the leaf structure could have—flex ear hybrids will still carry the higher risk."

Knowing that hybrids will have a bell curve to population, Ferrie says farmers should use that knowledge in their own test plots.

"When experimenting with populations and row spacings, farmers need to start low and work their way up. Go above and go below your normal planted populations," he says.